Bradymonabacteria, a novel bacterial predator with versatile survival strategies in saline environments Da-Shuai Mu Shandong University Shuo Wang Shandong University Qi-Yun Liang Shandong University Zhao-Zhong Du Shandong University Renmao Tian University of Oklahoma Yang Ouyang University of Oklahoma Xin-Peng Wang Shandong University Aifen Zhou University of Oklahoma Ya Gong Shandong University Guan-Jun Chen Shandong University Joy Van Nostrand University of Oklahoma Yunfeng Yang Tsinghua University Jizhong Zhou University of Oklahoma Zongjun Du ( [email protected] ) Shandong University https://orcid.org/0000-0002-7886-5667 Research Keywords: Bacterial predator, Bradymonadales, Metabolic deciencies, Comparative genomic analysis, Biogeographic analysis Posted Date: January 9th, 2020 DOI: https://doi.org/10.21203/rs.2.20535/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Page 1/20 Version of Record: A version of this preprint was published on August 31st, 2020. See the published version at https://doi.org/10.1186/s40168-020-00902-0. Page 2/20 Abstract Background: Bacterial predation is an important selective force for microbial community structure and dynamics. However, only a limited number of predatory bacteria were reported, and their predatory strategies and evolutionary adaption remain elusive. We recently isolated a novel group of bacterial predator, Bradymonabacteria, representative of the novel order Bradymonadales in δ-Proteobacteria. Compared with other bacterial predators (e.g. Myxococcales and Bdellovibrionales), the predatory and living strategies of Bradymonadales remain largely unknown. Results: Bradymonabacteria preyed on diverse bacteria but had a high preference on Bacteroidetes based on individual co- culture of Bradymonabacteria with 281 prey bacteria. Genomic analysis of 13 recently sequenced Bradymonabacteria indicated that these bacteria had conspicuous metabolic deciencies, but they could synthesize many polymers, such as polyphosphate and polyhydroxyalkanoates. Dual-transcriptome analysis of co-culture of Bradymonabacteria and prey suggested the potential contact-dependent predation mechanism. Comparative genomic analysis with other 24 bacterial predators indicated Bradymonabacteria had different predatory and living strategies. Furthermore, we extracted Bradymonadales from 1552 public available 16S rRNA amplicon sequencing samples and found that Bradymonadales was widely distributed and highly abundant in saline environments. Phylogenetic analysis showed there may be six proposed subgroups for this order and each subgroup occupied a different habitat. Conclusions: Bradymonabacteria have unique living strategies, which are different from so-called “obligate” or “facultative” predators. We propose a framework to categorize the current bacterial predators into 3 groups: (i) highly prey-dependent predators, (ii) facultative prey-dependent predators, and (iii) prey-independent predators. Our ndings provide an ecological and evolutionary framework for Bradymonadales and highlight their potential ecological roles in saline environments. Background Bacterial predators are proposed as an indispensable selective force for bacterial communities [1–4]. Predation by bacteria could release nutrients [5] and affect biogeochemical cycling. In contrast to phages, bacterial predators do not need to be in high concentrations to drive signicant bacterial mortality in the environment [6, 7]. And bacterial predators have a higher eciency to kill prey in low-nutrient medium compared with phage [8]. However, these studies mostly based on the predators Bdellovibrio and like organisms (BALOs), little ecological roles are known on other bacterial predators. Predatory bacteria are classied as two categories, obligate or facultative predators, based on their prey independent/dependent living strategies [9]. Obligate predators are constituted of several genera collectively known as BALOs [10]. These predatory bacteria can attack their prey by penetrating the cell wall [11], dwelling in the periplasm and then killing their quarry [12]. Therefore, their lifestyle depends on their prey in environments, and BALOs lose viability within several hours if prey is not available [8, 13]. Facultative predators contain several genera [9], such as Myxococcus, Lysobacter, and Herpetosiphon [14]. These predators kill their prey by secreting antimicrobial substances into the surrounding environment [9]. In general, facultative predators were considered as they can be cultured as bacteria pure culture. However, the obligate predators could also be pure cultured on complex microbial extract-based media [15]. Indeed, most so-called obligate predators have a host-independent lifestyle [12, 16]. As a result, the denition “obligate predator” doesn’t fully describe their lifestyle. Thus, it is needed to develop a framework to categorize the current bacterial predators. Bradymonabacteria are representative of the novel order Bradymonadales, which phylogenetically located in the δ- Proteobacteria [17]. The rst type species of Bradymonadales was isolated in 2015 [17] (i.e., Bradymonas sediminis FA350T). To date, 9 strains within the Bradymonadales have been isolated belonging to 7 candidate novel species, and Bradymonabacteria are also bacterial predators [18]. Interestingly, the δ-Proteobacteria contain three orders of predatory bacteria, Myxococcales, Bdellovibrionales (also categorized into Oligoexia in 2017 [19]), and Bradymonadales. Myxococcales and Bdellovibrionales are facultative and obligate predators, respectively. Also, they have different distribution patterns in environment. Myxococcales were mainly found in soil and sediments niches [20, 21] while Page 3/20 Bdellovibrionales were aquatic. However, how Bradymonadales adapts to predation lifestyle, and whether they have specic living strategies or ecological importance remain largely unknown. Here, we analyzed Bradymonadales predation range on diverse bacteria and their predatory morphological and physiological characteristics. By using comparative genomic analysis of Bradymonadales and other predatory bacteria, we revealed the genetic and metabolic potential of this group. To assess the diversity and frequency of occurrence of the various ribotypes of known predators (Bradymonadales, Myxococcales and Bdellovibrionales) on a global scale, we surveyed published 16S rRNA gene amplicon datasets from a number of ecosystems representing a broad range of geographic location, climatic zone, and salinity. Our study provides an ecological and evolutionary framework for Bradymonadales and highlights their potential ecological roles in predation. Results Bradymonabacteria are ecient predators of diverse prey bacteria Totally, 9 strains of bacteria with in the novel order Bradymonadales have been isolated by using enrichment culture method [22]. Among these strains, strain FA350T [17, 18] and B210T [23] were the two type strains within different genera in Bradymonadales. And both type strains were used to investigate the predator-prey range of Bradymonabacteria. A total of 281 isolated bacteria were co-cultured with Bradymonabacteria FA350T [17, 18] or B210T [23] as lawns on Petri dish, respectively (Fig. 1a, Table S1). Zones of predation were measured (Fig. 1b), and results showed Bradymonabacteria preyed on diverse bacteria but a high preference on Bacteroidetes (90% of tested bacteria could be preyed) and Proteobacteria (71% of tested bacteria could be preyed) (Fig. 1c). Predation on bacteria in the orders Flavobacteriales, Caulobacterales, Propionibacteriales, and Pseudomonadales were broadly distributed, with the mean predation percentage greater than 90%, while, predation of Micrococcales and Enterobacteriales were less ecient. Transmission electronic microscopy (TEM) and scanning electronic microscopy (SEM) analyses were done to understand the mechanism of predation of strain FA350T on subcellular level. Lysis of the prey cells can be detected nearby strain FA350T in both TEM and SEM analyses (Fig. 2). Strain FA350T was detected to have pili (Figs. 2b and 2 g) and the outer membrane vesicles (OMVs) like structures (Figs. 2d, 2e, 2f, and 2 h). In addition, FA350T cells contained intracellular particles with low electron-dense (Figs. 2b, 2c, 2d, and 2f), which was polyhydroxyalkanoates (PHAs) tested by Nile blue A staining. Meanwhile, FA350T cells also contained several electron-dense intracellular granules (black granules) (Fig. 2b, 2c, 2d, and 2f), which indicated the presence of intracellular polyphosphate granules [24]. Both these particles were signicantly accumulated during predation (Fig. 2). Bradymonabacteria are multiple auxotrophs To explore the metabolic capabilities and predation mechanism of this novel group, we analyzed 13 genomes of Bradymonadales (9 high-quality genomes sequenced from cultured strains and 4 reconstructed from published studies [25]). Genome size of Bradymonabacteria ranged from 5.0 Mb to 8.0 Mb. Average Nucleotide Identity (ANI) analysis of the 9 cultured strains of Bradymonadales revealed 7 different species [26] (Fig. S1b). Other General features of genomes were described in Supplementary Materials (Supplementary Materials Results and Fig. S1a). Almost all strains (except FA350T) possessed a minimal Pentose Phosphate Pathway, which lacked key steps for the synthesis of ribose 5-phosphate (Fig. 3, Table S2) [27]. Most of the Bradymonabacteria genomes lacked key enzymes for pyrimidine synthesis,